Long ago, in a galaxy far, far away, there may have been someone who had not heard of “the dark side.” But I seriously doubt it. Whether you are a Star Wars aficionado or not, there is no doubt that these hugely popular movies have saturated the culture and the common vocabulary. As Jedi knight Obi-Wan Kenobi explained to the innocent young Luke Skywalker in the original 1977 film, the once-virtuous Darth Vader was seduced by the dark side of the Force, his destructive power fueled by rage and hate. The dark side is all the galaxy’s evils rolled together.

Of course, Star Wars creator George Lucas did not invent the metaphor of the dark side. Not even remotely. From Shakespeare to the Bible and other sacred texts, darkness has been inextricably linked with malevolence and condemnation, and light with goodness. But why? Where does this seemingly universal metaphor come from, and how does it shape our psychology?

One popular idea is that metaphors are rooted in basic sensory experience. The dark side may help us understand and communicate abstract ideas like immorality and evil, but the metaphor gets its power from our actual perceptions of the world around us. At least that’s the theory, which University of Kansas psychological scientist Promothesh Chatterjee and his colleagues decided to put to the test in a couple of simple laboratory experiments.